In recent years, as the performance of electronic devices improves and portable electronic devices become popular, an SOI (Silicon On Insulator) wafer adsorbs attention which can operate at a higher speed with lower power consumption than a bulk wafer.
Methods of manufacturing SOI wafers are roughly classified into an SIMOX method and bonding method. The bonding method is superior to the SIMOX method in that it can use a high-quality silicon thermal oxide film as a buried insulating layer. According to the bonding method, typically, a thermal oxide film is formed on the surface of one or each of two silicon wafers by the thermal oxidation method, and thereafter the two silicon wafers are bonded. Then, the bonding strength of the wafers is increased by annealing. Furthermore, unnecessary portions are removed to leave an SOI layer having a desired thickness. The bonding method can control the thicknesses of the SOI layer and buried insulating layer freely.
A typical example of the bonding method includes a method described in Japanese Patent Laid-Open No. 5-21338 (this method is known as the ELTRAN (registered trademark) method) and a method described in Japanese Patent Laid-Open No. 5-211128 (this method is known as the Smart Cut (registered trademark) method).
As the bonding method includes a step of bonding two wafers, as described above, defects such as voids should not be caused in the bonding interface. Various types of approaches have been made to decrease the defects in the bonding interface.
Japanese Patent Laid-Open No. 2002-313689 discloses a method of bonding two substrates as bonding targets while leaving moisture contents on the two substrates to decrease void defects or blister defects. According to the prior art introduced in Japanese Patent Laid-Open No. 2002-313689, the two substrates as the bonding targets are cleaned, and before they are bonded, they are dried by IPA steam dry.
Japanese Patent Laid-Open No. 2003-309101 discloses a method of SC-1-cleaning two substrates and thereafter bonding them.
Japanese Patent No. 3294934 discloses a method of activating, before bonding two substrates, the surface of one substrate by a plasma process and thereafter adsorbing water with the activated surface.
Japanese Patent Laid-Open No. 9-331049 discloses a method of rendering, before bonding two substrates, the surface of one substrate hydrophobic.
In bonding the two substrates, a high bonding strength is obtained if the bonding step is performed after the bonding surfaces are rendered hydrophilic. As discussed in Japanese Patent Laid-Open No. 9-331049, however, if the bonding surfaces are rendered hydrophilic, due to excessive moisture contents adsorbed by the bonding surfaces, voids tend to be easily formed in the peripheral portions of the bonding surfaces. In view of this, Japanese Patent Laid-Open No. 9-331049 proposes an approach in which a bonding step is performed after the bonding surfaces are rendered hydrophobic and thereafter the lack in bonding strength is compensated for by a later annealing step.
In other words, conventionally, when the bonding surfaces are imparted with hydrophily, voids tend to be formed in the peripheral portion of the bonding interface. When the bonding surfaces are imparted with hydrophoby, the bonding strength tends to decrease.